Whelahan lamented a sluggish first half display which ultimately left Birr with too much to do after half-time.
He said: "The first-half performance wasn't a Birr performance, not the way we trained and the way we put things together.
Despite their poor first half, Birr arguably created enough chances in the second half to win the game.
Antrim's hurling kingpins certainly owe opponents Birr a beating having lost out to the Offaly men in the 1995 All-Ireland club final replay and then suffering another narrow defeat last year.
Birr will start as hot favourites to land a record fourth All-Ireland in a decade - but McLean believes there will be a shock on the cards.